Recently, recorders capable of recording moving image data in a hard disc or a DVD (Digital Versatile Disc) are coming along and replacing conventional VCRs for recording TV programs. These recorders can record enormous amounts of moving image data on a single disc by compressing the data using efficient encodings such as MPEG (Moving Picture Experts Group) 2.
Efficient moving image encoding as represented by MPEG2 generally executes compression encoding by using a plurality of different picture types including intra-frame and inter-frame prediction. More specifically, only frames at a predetermined interval use a picture type for intra-frame encoding. Frames between them use a picture type for inter-frame prediction due to the fact that successive frames have high image data correlation, thereby increasing the moving image data compression efficiency.
FIG. 17 is a block diagram showing a conventional image signal processing apparatus. Reference numeral 1702 denotes a camera unit; and 1701, a camera including a optical lens system and a photo-electric conversion unit such as a CCD (Charge Coupled Device). An analog moving image signal photographed by the camera 1701 undergoes processing such as A/D-conversion, pixel interpolation, color conversion, and γ-conversion by a photographed image signal processing circuit 1703. A camera control circuit 1725 executes processing such as exposure control of the camera 1701 on the basis of, for example, the brightness of the photographed image. If a dark image will be photographed at a the shutter speed less than a predetermined value in the moving image photographing mode of the camera unit 1702, the camera control circuit 1725 raises the amplifier gain of the photographed image signal processing circuit 1703 in order to not reduce the shutter speed, thereby preventing any afterimage.
The photographed image signal processing circuit 1703 supplies its output to a frame rearranging circuit 1709 as image data to be encoded. The frame rearranging circuit 1709 rearranges frames in encoding order. As an example of this, in an MPEG2 encoding, the frames are rearranged in an order suitable for encoding. For example, a B-picture serving as a bidirectional prediction frame should be encoded after the encoding of the preceding and succeeding frames and therefore it is moved backward.
For I-pictures, a difference circuit 1710 outputs the image data itself to a DCT circuit 1711. For P- and B-pictures, the difference circuit 1710 calculates the difference value between the image data and a predicted image and outputs the difference value to the DCT circuit 1711 according to a selection by a switch 1723. The DCT circuit 1711 converts the image data into a DCT coefficient. A quantization circuit 1712 quantizes the DCT coefficient using a predetermined quantization scale Q. When the Q value changes, the coefficient value after quantization changes greatly. Hence, the generated code amount changes.
A variable-length coding circuit 1713, for example, entropy-encodes the quantization coefficient output from the quantization circuit 1712 and outputs it as encoded data. A buffer 1714 temporarily saves the generated encoded data to control the encoding rate. The encoded data stored in the buffer 1714 is read out at a predetermined rate and output from a terminal 1715 as compression-encoded data.
Meanwhile, the coefficient data quantized by the quantization circuit 1712 undergoes inverse quantization by an inverse quantization circuit 1717 and inverse DCT by an inverse DCT circuit 1718 to obtain predicted image data. For I-pictures, an adding circuit 1719 saves data after inverse DCT directly in a video memory 1720. For P- and B-pictures, the adding circuit 1719 adds the predicted image to the P- and B-pictures and saves it in the video memory 1720 as locally decoded image data. A motion compensation predicting circuit 1721 compares the locally decoded image data saved in the video memory 1720 with the input image data. For P-pictures, predicted image data with motion compensation in the forward direction is generated and supplied to the above-described difference circuit 1710. For B-pictures, predicted image data with bidirectional motion compensation is generated and supplied to the above-described difference circuit 1710. The motion compensation predicting circuit 1721 also supplies the generated image data to the adding circuit 1719 as a predicted image for next local decoding.
A rate control circuit 1727 executes code amount assignment control of a picture to be encoded for a target encoding rate using information such as a past generated code amount and buffer fill factor obtained from the buffer 1714. At this time, the rate control circuit 1727 controls the quantization circuit 1712 by deciding the quantization scale Q based on a code amount assigned to each picture type. The generated code amount after quantization changes between the I-, P-, and B-pictures. The quantization scale Q is therefore generally changed in accordance with the picture type. For example, an encoding scheme known as MPEG2-TM (Test Model) sets the quantization scale corresponding to each picture type.
Conventional video cameras execute exposure control of the camera 1701 in accordance with the brightness of a photographed image, as in the above-described camera unit 1702. In a normal brightness range, the video camera generally maintains adequate exposure mainly by combining the F-number and shutter speed. However, a video camera may increase the gain in a dark scene with a full aperture. Generally, an image photographed at the increased gain has a low S/N ratio because of a random noise component mixed into the image. In this case, if an encoding such as the above-described MPEG2 using a plurality of different picture types is used, the luminance peak generated by the noise component after encoding varies depending on the picture type. The variation is observed as a luminance flicker in the reproduced moving image, resulting in a visual disturbance, as is known.
Under these circumstances, a patent proposal has been made to remove random noise generated when increasing the gain by improving the characteristic of a filter circuit synchronized with increases in the gain (for example, WO97/05745).
FIG. 16 shows graphs a through c for explaining the cause of a luminance flicker in the reproduction mode which is generated when increasing the gain of the camera used in the conventional arrangement described above. The graph a of FIG. 16 shows a reproduction signal from I-pictures. The luminance peak of the noise component superimposed on a flat image signal is maintained to some extent by intra-frame encoding. This is also because the assigned code amount of I-pictures is generally larger than that of the other picture types.
The graph b of FIG. 16 shows a reproduction signal from P- and B-pictures. In an image photographed at an increased gain, the correlation between frames is low because of the random noise component. In normal encoding, therefore, the encoded image signal degrades because of the increase in inter-frame difference information of P- and B-pictures. The noise peak decreases so that a luminance peak difference is generated, unlike the I-pictures in the graph a of FIG. 16. Hence, a luminance flicker derived from noise is generated in reproducing a moving image, as shown in the graph c of FIG. 16.
The conventional technique to remove random noise derived when increasing the gain, i.e., the method of changing the filter characteristic as in WO97/05745 is one of general encoding distortion reducing methods in the case of a low S/N ratio and can expect a partial effect. However, in order to completely remove the above-described luminance flicker using a filter alone, the filter intensity must be sufficiently high. However this considerably degrades the resolution and causes serious, adverse effects such as an afterimage in the original image. A TV program image can sometimes contain intentionally added noise intended to produce a film-like effect based on the granularity of noise. In this situation, it is impossible to uniformly remove noise using a filter. Removal of the noise component itself must be suppressed as much as possible. That is, to effectively reduce the above-described luminance flicker while maintaining image quality, the invention of WO97/05745 does not suffice.